第4条性质： \begin{split} &\ &\vec{a} \parallel \vec{b} \\ \Rightarrow &\ & \vec{a}=\lambda \vec{b} \\ \Rightarrow &\ & \left[\begin{array}{l}x_{1} \\ y_{1}\end{array}\right] = \lambda \left[\begin{array}{1}x_{2} \\ y_{2}\end{array}\right] \\ \Rightarrow &\ & \left\{\begin{array}{l}x_{1}=\lambda x_{2} \\ y_{1}=\lambda y_{2}\end{array} \right. \\ \Rightarrow &\ & x_{1}*\lambda y_{2} = y_{1}*\lambda x_{2} (上式交叉相乘) \\ \Rightarrow &\ & x_{1}y_{2}-x_{2}y_{1}=0. \end{split}